The present invention concerns a power supply circuit with a self-oscillating blocking oscillator, which, with the help of a switching transistor and a transformer, produces a DC voltage which can be taken from the transformer via a rectifier arrangement from the DC which is fed to the blocking oscillator. A regulating circuit is also provided in the control circuit of the switching transistor which controls the switching transistor as a function of a comparison voltage which is drawn from the primary circuit of the transformer.
Power supply circuits of this type are, for example, known from German Pat. No 2,160,659 incorporated herein by reference. In the case of such known power supply circuits, the drive signal for the regulating circuit which triggers the switching transistor is created by means of a resistor in the emitter circuit of the switching transistor. In the case of a design of this sort, nevertheless the disadvantage occurs that in the case of a failure of the switching transistor, the complete rectified supply voltage occurs at the emitter resistor, so that the drive circuit can be destroyed. Further, the voltage at this emitter resistor is also burdened with overshoot components, so that hunting arises.